Cascaded steam temperature control applied to a universal pressure boiler

ABSTRACT

The firing rate and the feedwater flow rate of a universal pressure boiler are controlled using a cascaded steam temperature control system. The secondary superheater outlet temperature comprises an outer loop for the control system, while the convection pass outlet temperature comprises an inner loop for the control system. Control variables for the system include the secondary superheater outlet temperature, the convection pass outlet temperature and the difference between the primary superheater outlet temperature and the secondary superheater inlet temperature.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates in general to control systems and inparticular to a new and useful cascaded control system for a universalpressure boiler.

A known method of steam temperature control for universal pressureboilers, as illustrated in FIG. 1, utilizes a furnace gas temperature(FGT) 2, a primary superheater outlet temperature (PSHOT) 3, and asecondary superheater outlet temperature (SSHOT) 4 as primary controlledvariables. This known control scheme utilizes a feedforward programbased on a feedwater temperature error (FWTE) 22, a secondarysuperheater inlet temperature (SSHIT) 23, a secondary superheater outlettemperature error (SSHOTE) (not shown) and a primary super heater outlettemperature error (PSHOTE) 21.

The known steam temperature control system illustrated in FIG. 1 isdevised to maintain a secondary superheater outlet temperature at a setpoint in order to account for transient disturbances within theuniversal pressure boiler. Currently, the conventional control systemfor the universal pressure boiler utilizes the furnace gas temperaturetransmitter 2, the secondary superheater outlet temperature transmitter3 and a primary superheater outlet temperature transmitter 4 as theprimary controlled variables. Unit load demand 20 is input into afunction generator 10 having a maximum gas temperature. A differenceunit 12 performs a subtracting action of the furnace gas temperature 2and the set program of furnace which is in function generator 10 gastemperature and provides this difference to a low select auctioneer 13for taking the lower value of the two.

Unit load demand 20 is input into a second function generator 110 forcontrolling the set program of secondary superheater outlet temperature.A second difference unit 112 performs a subtraction action on thesecondary superheater outlet temperature 3 and the unit load demand 20from the set program of the secondary superheater outlet temperaturefunction generator 110. This difference is provided to a high selectauctioneer 14 for taking the higher value of the two. Unit load demand20 is input to a third function generator 210 for providing the desiredprimary superheater outlet temperature. This difference or primarysuperheater outlet temperature error 21 is provided to the high selectauctioneer 14. The primary superheater outlet temperature error 21 isalso provided to the low select auctioneer 13 in conjunction with thedifference from the first difference unit 12.

The low select auctioneer 13 provides the lower value i.e. thedifference from the difference unit 12 or the primary superheater outlettemperature error 21, to a derivative action unit 15. The lower valuefrom the low select auctioneer 13 is also provided to a summer 17 and atransfer action unit 16. Derivative action unit 15 performs a ratefunction upon the value from the low select auctioneer 13. This rateprovided by the derivative action unit 15 is summed along with the valuefrom the low select auctioneer 13 by the summer 17. After a summingaction is performed by the summer 17, this value is provided to a secondsummer 117.

The high select auctioneer 14 takes the greater value from either theprimary superheater outlet temperature error 21 or the value provided bythe second difference unit 112. The greater value is provided by thehigh select auctioneer 14 to the transfer action unit 16. The value fromthe high select auctioneer 14 is also provided to a second derivativeaction unit 115 for performing a rate function which is in turn providedto the second summer 117.

The transfer action unit 16 performs a transfer action of the value fromthe low select auctioneer 13 and the high select auctioneer 14 whichconstitutes a low load or bypass selection and once-through operation.Transfer action unit 16 transfers these values to an integral actioncontrol unit 18 which performs an integral function on these values andprovides the result to the second summer 117.

Second summer 117 performs a summing action on values from the secondderivative action unit 115, the integral action control unit 18, thefirst summer 17, a feedwater temperature error 22 and a secondarysuperheater inlet temperature 23 supplied through a third derivativeaction unit to 215.

After performing the summing action at the second summer 117, this valueis provided to a multiplier 19 for performing a multiplying action. Theunit load demand 20 is provided through a fourth function generator 310to the multiplier 19 for multiplication with the value from the secondsummer 117.

The value from the multiplier 19 is provided to an inverse proportionalaction unit 11 and a fourth summer 317. Fuel flow demand 24 is alsoprovided to the third summer 217 which in turn performs a summing actionon the loss from the proportional action unit 11 and the fuel flowdemand 24 for determining a firing rate 30 for the universal pressureboiler.

The fourth summer 317 takes the value from the multiplier 19 along witha feed water flow demand 25 for performing a summing action. Due to thesumming action performed on these Values, a feedwater flow rate 31 forthe universal pressure boiler is determined.

The conventional steam temperature control strategy, as described above,is divided into main regions: a low load or bypass region and anoperating or high load region. In the low load region, which isgenerally the low range before a transfer is conducted from a flash tankoperation or drum boiler type to a once-through operation, the furnacegas temperature 2 is used as the controlled variable. The set point isthe load base 20 which is characterized for temperature. In the low loadregion, the primary superheater outlet temperature error 21 is used asan overriding signal for the furnace gas temperature error when themaximum temperature operating limit is reached near the minimumfeedwater flow load. The overriding circuit of the conventional controlsystem also provides protection for the superheater tubes.

The operating or high load region of the universal pressure boilercovers the load point range from a once-through operation to a full loadoperation. In the high-load region, it is necessary to use the primarysuperheater outlet temperature 4 and/or the secondary superheater outlettemperature 3 as the controlled variables. At this stage, temperatureprobes used in conjunction with the universal pressure boiler areretracted in order to render the furnace gas temperature control 3 as anon-factor.

The set point for the high load region is generated as a function ofload characterized as steam temperature. A secondary superheater outlettemperature error is created by comparing the secondary superheateroutlet temperature 3 to the programmed set point. This error is limitedby the primary superheater outlet temperature error 21 which acts as anoverride circuit and a protector for the superheaters in case of extremetemperature excursions.

During a high load range, this scheme provides for transient conditionsby using transient factors. One such transient factor is the feedwatertemperature error 22 wherein an increase of this error implies areduction in the superheater temperature because for any given steamflow, less fuel is fired into the boiler and less gas passes over thesuperheaters.

Another transient factor is a secondary super-heater inlet temperaturekicker which is a signal used for adjusting the feedwater to fuel ratiowhich is caused by a change in the operating condition reflected at thesecondary superheater inlet temperature 23 before the secondarysuperheater outlet temperature 3. Another transient factor is thesecondary superheater outlet temperature error wherein its rate ofchange is indexed as a function of load and is used to correct thefeedwater rate and firing rate.

There are several problems involved with employing the conventionalsteam temperature control as described above. First there is instabilitywithin the boiler at low load operations when controlled by the furnacegas temperature variable 2. Second, there is a lack of control andunaccountability for the tremendous dead time that exists at low loadsdue to the high heat absorption. Third, there is limited adaptivecontrol capability for the system since the system controls only thefinal element i.e. the secondary superheater outlet temperature 3.Fourth, there is a slow or sluggish response in the operating range andovercompensation or undercompensation can occur due to the numerouskickers that are used as anticipatory signals. Fifth, the secondarysuperheater outlet temperature 3 is used as a kicker but in reality thissignal is a feed-back and if not used with moderation, can create acontinuous cycling problem for the system.

SUMMARY OF THE INVENTION

The present invention provides for a cascaded steam temperature controlfor a universal pressure boiler. The combination of signals used by thesystem of the present invention to provide temperature control are thesecondary superheater outlet temperature, the convection pass outlettemperature, and the difference of the primary superheater outlettemperature with the secondary superheater inlet temperature. Thecontrol system of the present invention utilizes the secondarysuperheater outlet temperature, convection pass outlet temperature,primary superheater outlet temperature and secondary superheater inlettemperature as the controlled variables. With a change in temperaturethe secondary superheater outlet temperature will move slower than theconvection pass outlet temperature. The present invention uses thesensitivity of the convection pass outlet temperature to predict thedirection and amount of change in the secondary superheater outlettemperature.

It is an object of the present invention to provide a steam temperaturecontrol system for a universal pressure boiler that is more efficientand cost effective than the conventional control systems.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a known control system for a universalpressure boiler.

FIG. 2 is a schematic diagram of a control system according to thepresent invention.

FIG. 3 is a chart plotting the temperature profile of the unit load ofthe present invention.

FIG. 4 is a chart plotting the feedwater and fuel ratio of the presentinvention.

FIG. 5 is a chart diagramming a process model according to the presentinvention.

FIG. 6 is a table illustrating the temperature profile for thecontrolled variables according to the present invention.

FIG. 7 are graphs illustrating the controlled variables with time.

FIG. 8 are continuations of the graphs shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the present invention, the same reference numerals are used todesignate similar elements utilized by known control systems for auniversal pressure boiler.

The present invention embodied in FIG. 2 shows that the secondarysuperheater outlet temperature 3 comprises an outer and slower loop forthe control system while a convection pass outlet temperature control 6is used as an inner and faster loop. The cascaded steam temperaturecontrol loop of the present invention provides for a steam temperatureset point 1 to be utilized for the system. Set point 1 from analogcontrol station 50 can be set to bias programmed set point 10.

A low select auctioneer 114 selects either the set point 14 or the setpoint 26, whichever is less and provides this value to a difference unit12. Primary superheater outlet temperature error 21 combined with setpoint 14 is used as an override signal in order to protect the secondarysuperheater outlet tubes of the boiler.

The secondary superheater outlet temperature transmitter 3 transmits itsvalue to difference unit 12. The value from the difference unit 12 ispassed to a first Smith Predictor function 40. As is well known to thoseskilled in the art, the Smith Predictor function performs a first orderfunction to thereby provide predictive process control from an errorsignal developed from the process variable and set point inputs measuredagainst an internal model of the process. The value from the SmithPredictor function 40 is provided to a summer unit 117.

The following characterized signals input to the summing unit 117 form afeedforward system:

Unit load demand 20 input to a function generator 110 whichcharacterizes it as a function of temperature.

Secondary superheater inlet temperature input to derivative action unit15, which output is gained by summer input 17.

The difference value of the primary superheater outlet temperature withthe secondary superheater inlet temperature.

Gas recirculation fan damper position 28.

Output of the Smith Predictor function 140.

Differential action unit 112 receives the summation value 117 inaddition to a convection pass outlet temperature 6 as another controlledvariable. The difference determined by the differential action unit 112is provided to a second Smith Predictor function 140. After theperformance by the Smith Predictor function 140, this value is providedto the feedwater flow rate and firing rate analog control station 50.

After analog control station 50 receives an adjustment signal from SmithPredictor function 140, it controls both the firing rate 30 and the feedwater flow rate 31. Analog control station 50 utilizes an inverteraction unit 111 for determining directional action to a multiplier 19for performing a multiplying action with fuel flow demand 24. Themultiplying action performed by the multiplier 19 provides the firingrate 30 for this system.

A second multiplier 119 performs a multiplying action upon the valueprovided by the analog control station 50 and feedwater demand 25. Thefeedwater flow rate 31 is determined by the multiplying action of themultiplier 119.

Key aspects of the control system according to the present inventioninclude set point generation, anticipatory signal generation, variableprocess limits, process modeling of the outer secondary superheateroutlet temperature loop using a first order approximation, errorcorrection/control action of the outer loop and the set point generationfor the convection pass outlet temperature 6, process modeling of theinner loop or the convection pass outlet temperature 6 using a firstorder approximation and error correction/control action on the innerloop and final control output.

The secondary superheater outlet temperature set point 1 is generated asa function of the unit load demand 20 which characterizes the steamtemperature profile for the universal pressure boiler. Set point 1 canbe biased by an operator in order to provide better temperature matchingduring a load change or upset condition. The primary superheater outlettemperature error 21 is used as an overriding signal for the programmedset point 14. Accordingly, this circuit is used to protect thesuperheater tubes and prolong the turbine life.

The load versus convection pass feedforward signal is determined byperforming a load ramp over the entire load spectrum of the universalpressure boiler. This load test determines natural characteristics ofthe convection pass outlet temperature 6.

FIG. 3 illustrates the temperature profile of the universal pressureboiler at various unit loads. The difference 29 in the primarysuperheater outlet temperature 4 and the secondary superheater inlettemperature 7 illustrated by ΔT is used as an anticipatory feedforwardsignal. The primary superheater outlet temperature 4 and secondarysuperheater inlet temperature signal 7 correction depends on its signand magnitude. For example, an increasing ΔT value reduces theconvection pass outlet set point which in turn sets the secondarysuperheater outlet temperature within the control range. A decreasing ΔTvalue has the reverse effect. This signal predicts enthalpy fluctuationsof the boiler and the forces that are used by the system for correctingthese fluctuations.

FIG. 3 illustrates the temperature profile for the feedwatertemperature, convection pass outlet temperature 6, primary superheateroutlet temperature 4, secondary superheater inlet temperature 7,secondary superheater outlet steam temperature and the primarysuperheater outlet steam temperature and secondary superheater inlettemperature difference 26.

The secondary superheater inlet temperature 7 is used as a kicker inthat fast derivative action is provided on the secondary superheaterinlet temperature 7 in order to achieve immediate correction on loadchanges. An increasing high value for the secondary superheater inlettemperature 7 induces a negative correction and a decreasing value forthe secondary superheater inlet temperature has an opposite effect.

The gas recirculation fan damper position 28 is used as an anticipatoryfeed forward signal in order to control the furnace heat absorption andthe steam temperature. It is important to note that a change in the heatabsorption pattern causes the secondary superheater outlet temperature,as one of the variables of the system, to be effected. Increasing thegas recirculation fan damper position 28 causes a decrease in thefurnace heat absorption which in turn causes an increase in theconvection pass outlet temperature 6, primary superheater outlettemperature 4 and the secondary superheater outlet temperature 3. Inorder to counteract this action, the convection pass outlet temperature6 set point must be decreased as a function of an increase in the gasrecirculation fan damper position 28 in order to help in maintaining thesecondary superheater outlet temperature 3 within the operating range.Reverse action is necessary if there is an decrease in the gasrecirculation fan damper position 28.

Variable process limits are utilized by the present invention in orderto allow for a more flexible system for preventing anti-windupsituations by varying the high and low limits of the controller based onfeedforward signals.

FIG. 5 illustrates a process model of the present invention using afirst order approximation. In order to arrive at the model of FIG. 5, aload ramp is performed over the entire boiler range. A predeterminednumber of step changes, typically five or six, are induced fordetermining the following variables: process gain, dead time, lag timeand time constant.

The process gain of the present invention is a change in the controloutput divided by the change in the process variable, i.e. the processgain is analogous to the sensitivity of the process. By way of example,the gas recirculation fan damper position 28 is introduced as an outsidecontributing factor which has a tremendous impact on the sensitivity ofthe process especially at low to medium ranges. Dead time for thepresent invention is the measure of the time needed for a change in theprocess variable to take effect after a change in the control output hasoccurred. Lag time is a measure of the time from the end of the processdead time to approximately 63% of the crest of the final process change(1 tau).

The time constant can provide either a derivative action or additionalfiltering or even lag time. A number equal to the lag time nullifiesthis function, while a number greater than the lag time providesadditional lag time; and a number less than the lag time provides aderivative action.

The present invention provides for corrective action to be taken on thesecondary superheater outlet temperature 3 by using a set point 14 thatis generated by a model. Additionally, the present invention providesfor control action of the convection pass temperature set point 6,wherethe secondary superheater outlet temperature generates the setpoint for the convection pass outlet temperature. Additional correctionsare performed by the feedforward program.

Moreover, the present invention allows for process modeling of the innerloop or convection pass outlet temperature 6 using a first orderapproximation. The inner loop or convection pass outlet temperature loop6 is more sensitive than the outer loop. The inner loop involves asmaller gain i.e. more sensitivity, and also utilizes dead time, lagtime and a tuning time constant with derivative action similar to theprocess modeling of the outer loop as previously described.

The present invention allows for a controller to maintain the convectionpass outlet temperature 6 at set point. The convection pass outlettemperature 6 must be maintained at its set point in order to controlthe secondary superheater outlet temperature 3. If the convection passoutlet temperature 6 is not maintained at its set point, the control ofthe secondary superheater outlet temperature 3 is virtually impossible.

FIG. 6 illustrates data gathered for a temperature profile forcontrolled variables of the present invention. The controlled variablesinclude the unit load demand 20, feedwater temperature, convection passoutlet temperature 6, primary superheater outlet temperature 4,secondary superheater inlet temperature 7 and secondary superheateroutlet temperature 3. Unit load demand 20 is in megawatts (mw) while theother controlled variables are in degrees fahrenheit (°F).

FIG. 6 shows that from a load range of 114.4 mw to 360.9 mw,temperatures are maintained well within control limits. A change in theunit load demand 20 from 157 mw to 170.75 mw provokes an increase of thesecondary superheater inlet temperature 7 and the secondary superheateroutlet temperature 3 and a counteracting move by the present inventionwhich forces the convection pass outlet temperature 6 and the primarysuperheater outlet temperature 4 to decrease in order to bring thesecondary superheater outlet temperature 3 to the set point 14.

During this change, the secondary superheater inlet temperature 7 isdeviated by +10° F. to 660° F.; the secondary superheater outlettemperature 3 is deviated by +6° F. to 1,006° F.; the convection passoutlet temperature is deviated by 4° F. to 746° F.; and the primarysuperheater outlet temperature 4 is deviated by 2° F. to 828° F. Thepresent invention allows for the secondary superheater outlettemperature 3 or the secondary superheater outlet temperature to remainwithin a +/-10° F. range during load changes.

The steam temperature control provided by the present invention isfurther illustrated by the graphs shown in FIGS. 7 and 8 of the trend inthe secondary superheater inlet temperature (graphs 7a and 8a), thesecondary superheater outlet temperature (graphs 7b and 8b), primarysuperheater outlet temperature (graphs 7c and 8c), the unit load demand(graphs 7d and 8d) and the convection pass outlet temperature (graphs 7eand 8e) when the unit load demand was changed as described above from157 Mw to 170.75 Mw. It should be appreciated that the horizontal axisin FIGS. 7 and 8 represents time and that the graph in FIG. 8 which hasthe same letter as a graph in FIG. 7 is simply a continuation of thatgraph with time. Each trend graph starts at time T0 (see FIG. 7) andends at time T6 (see FIG. 8). In order to provide continuity between thegraphs shown in FIGS. 7 and 8, the graphs of FIG. 8 overlap the graphsof FIG. 7 for times T2, T3 and T4.

FIG. 4 illustrates the feedwater and fuel flow ratio and theirrelationship with the secondary superheater outlet temperature.

The present invention provides for tight control of the steamtemperature of the universal pressure boiler which is crucial for theplant operation and life of the equipment. Benefits provided by thepresent invention include an improvement of the unit heat rate;protection of the superheater tubes; prevention of thermal expansion anderosion of the turbine; control of temperature fluctuations; andapplication for variable pressure operation.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A system for controlling steam temperature of auniversal pressure boiler, said boiler including means for measuringtemperature at a primary superheater outlet, a secondary superheaterinlet, a secondary superheater outlet and a convection pass outlet ofsaid boiler and for measuring a unit load demand of said boiler, saidsystem comprising:means for providing a signal indicative of adifference between said measured primary superheater outlet temperatureand said measured secondary superheater inlet temperature; meansresponsive to said measured unit load demand of said boiler for derivinga setpoint for said secondary superheater outlet temperature; means forproviding a first Smith Predictor function for the difference betweensaid derived setpoint for said secondary superheater outlet temperatureand said measured secondary superheater outlet temperature; meansresponsive to said first Smith Predictor function, said unit loaddemand, said temperature difference signal and said measured secondarysuperheater inlet temperature for deriving a setpoint for saidconvection pass outlet temperature; means for providing a second SmithPredictor function for the difference between said derived setpoint forsaid convection pass outlet temperature and said measured convectionpass outlet temperature; and means responsive to said second SmithPredictor function for providing a signal for controlling a firing rateand a feedwater flow rate of said boiler to thereby control said boilersteam temperature.
 2. The system of claim 1 wherein said boiler includesmeans for measuring a fan damper position for said boiler and said meansresponsive to said first Smith Predictor function, said unit loaddemand, said temperature difference signal and said measured secondarysuperheater inlet temperature is also responsive to said measured fandamper position.
 3. A method for controlling steam temperature of auniversal pressure boiler, said boiler including means for measuringtemperature at a primary superheater outlet, a secondary superheaterinlet, a secondary superheater outlet and a convection pass outlet ofsaid boiler and for measuring a unit load demand of said boiler, saidmethod comprising the steps of:providing a signal indicative of adifference between said measured primary superheater outlet temperatureand said measured secondary superheater inlet temperature; deriving inresponse to said measured unit load demand of said boiler a setpoint forsaid secondary superheater outlet temperature; providing a first SmithPredictor function for the difference between said derived setpoint forsaid secondary superheater outlet temperature and said measuredsecondary superheater outlet temperature; deriving in response to saidfirst Smith Predictor function, said unit load demand, said temperaturedifference signal and said measured secondary superheater inlettemperature a setpoint for said convection pass outlet temperature;providing a second Smith Predictor function for the difference betweensaid derived setpoint for said convection pass outlet temperature andsaid measured convection pass outlet temperature; and providing inresponse to said second Smith Predictor function a signal forcontrolling a firing rate and a feedwater flow rate of said boiler tothereby control said boiler temperature.
 4. The method of claim 3wherein said boiler includes means for measuring a fan damper positionfor said boiler and said step of deriving in response to said firstSmith Predictor function, said unit load demand, said temperaturedifference signal and said measured secondary superheater inlettemperature a setpoint for said convection pass outlet temperature isalso responsive to said measured fan damper position.